A high-frequency signal belonging to a communication band such as a very high frequency (VHF) band, an ultra-high frequency (UHF) band or a microwave band is modulated according to a modulation method such as quadrature phase shift keying (QPSK) or quadrature amplitude modulation (QAM).
In general, an amplitude of the high-frequency signal is changed with time in every time period corresponding to a band of a modulation wave. In other words, an envelope of the high-frequency signal is changed with time. In cases where a plurality of signals are simultaneously amplified in a base station for mobile communication, an envelope of the high-frequency signal is changed with time.
FIG. 1 is a view showing an example of a waveform of a high-frequency signal of which an envelope is changed with time. An X axis indicates time, and a Y axis indicates electric power of a high-frequency signal.
As shown in FIG. 1, an envelope of a high-frequency signal is changed with time, and it is realized that a peak electric power of the high-frequency signal at a maximum envelope obtained in an instant is higher than an average electric power of the high-frequency signal. A ratio of the peak electric power to the average electric power is called a crest factor or a peak electric power ratio. The crest factor of the high-frequency signal used for a base station of recent mobile communication sometimes reaches 10 dB.
Therefore, in cases where it is intended to amplify a high-frequency signal having a high crest factor without saturating the crest factor at a peak electric power of the high-frequency signal, it is required to use a high-frequency amplifier in which a difference (so-called back-off) between an average output electric power at an actual operation of the high-frequency amplifier and a saturation electric power can be sufficiently heightened.
In general, in a high-frequency amplifier, an internal output matching circuit is adjusted according to required characteristics of a high-frequency signal, and output side load impedance conditions seen from an amplifying element such as a field-effect transistor (FET) are optimized. For example, there is a case where the load impedance conditions are optimized so as to heighten efficiency in the high-frequency amplifier, and there is another case where the load impedance conditions are optimized so as to heighten the saturation electric power.
However, as is described in a literature “Monolithic Microwave Integrated Circuit” (written by AIKAWA et al., the Institute of Electrics, Information and Communication Engineers, p. 74, FIGS. 3-4) or a literature “Basis and Applications of MMIC Technique” (written by TAKAGI and ITOU, published by REALIZE company, p. 155, FIGS. 8-39(a)), a load impedance condition (or high efficiency matching) corresponding to the high efficiency do not generally agree with a load impedance conditions (or high output matching) corresponding to the high saturation electric power.
FIG. 2 is a view schematically showing both a change of a saturation electric power and a change of an efficiency on a Smith chart with respect to a load impedance seen from an amplifying element of a high-frequency amplifier. Solid lines indicate contour lines of the saturation electric power, and dotted lines indicate contour lines of the efficiency.
In FIG. 2, a load impedance corresponding to a maximum saturation electric power is indicated by a symbol x, and a load impedance corresponding to a maximum efficiency is indicated by a symbol •. The load impedance corresponding to the maximum saturation electric power does not agree with the load impedance corresponding to the maximum efficiency, and the load impedance corresponding to the maximum efficiency is changed with respect to an output electric power.
Accordingly, a great portion of the instantaneous electric power of the high-frequency signal is cut off on the load impedance condition corresponding to the high efficiency. As a result, a large amount of disturbance wave is leaked to an adjacent channel, the transmitted high-frequency signal is degraded, and transmission error is increased.
In contrast, on the load impedance condition corresponding to the high saturation electric power, no great portion of the instantaneous electric power of the high-frequency signal is cut off, but the efficiency in the high-frequency amplifier is undesirably lowered.
Because of the above-described reasons, it is difficult to obtain a high-frequency amplifier in which the load impedance condition corresponding to the high saturation electric power and the load impedance condition corresponding to the high efficiency are satisfied. In other words, the efficiency in the high-frequency amplifier is considerably lowered on condition that the back-off is high. For example, in case of a simple class-B amplifier, a maximum efficiency in a saturation operation is theoretically equal to 78%, and a maximum efficiency in an operation of the back-off of 10 dB is theoretically equal to about 25%. Therefore, though a high-frequency amplifier of a base station is required to amplify a high-frequency signal of a high crest factor at low distortion, the efficiency in the high-frequency amplifier is lowered due to the low maximum efficiency in the operation at the back-off of 10 dB.
To solve the above-described problem, a conventional high-frequency amplifier is known, where the load impedance in the conventional high-frequency amplifier is changed by using a switch.
FIG. 3 is a view showing the configuration of a conventional high-frequency amplifier disclosed in the Published Unexamined Japanese Patent Application No. H9-284061 (1997), and FIG. 4 is a view showing the configuration of an output matching circuit used for the conventional high-frequency amplifier.
In FIG. 3, 101 indicates an amplifying element such as a field-effect transistor (FET) or a bipolar junction transistor (BJT). 102 indicates a power supply circuit for applying a direct-current voltage to the amplifying element 101. 103 indicates an output matching circuit. 104 indicates a load.
The output matching circuit 103 has the configuration shown in FIG. 4 as an example and comprises a transformer 105 for the impedance change and a switch 106 for changing over from a tap terminal of the transformer 105 to another tap terminal. A tap terminal selected by the switch 106 is changed to another tap terminal according to a control signal Sct1, and a value of a load impedance Zin obtaining by seeing the output matching circuit 103 from the amplifying element 101 is changed.
In cases where the output electric power is low, the switch 106 selects a tap terminal corresponding to a load impedance condition of the high frequency. In cases where an output electric power is high, the switch 106 selects a tap terminal corresponding to a load impedance condition of the high output electric power, that is, a load impedance heightening the saturation electric power. Therefore, an excellent efficiency can be obtained regardless of a value of the output electric power.
Also, similar high-frequency amplifiers are disclosed in the Published Unexamined Japanese Patent Application No. H5-54725 (1993) and the Published Unexamined Japanese Patent Application No. H11-41118 (1999). In the conventional technique of these applications, the selection in a switch of a matching circuit is changed, a load impedance is changed by using a variable capacitive element arranged in the matching circuit, and an excellent efficiency is obtained regardless of a value of the output electric power. Because the conventional high-frequency amplifier has the above-described configuration, a problem has arisen that it is difficult to obtain a high-frequency amplifier operated at a sufficiently high saturation electric power and an excellent efficiency while following the change of an envelop of a high-frequency signal.
As is described before, in the conventional technique, because the selector switch or the variable capacitive element is arranged in the conventional high-frequency amplifier, a reaction time for the change of the load impedance is long, and the load impedance cannot be changed in accordance with a time change of the envelope based on a modulation wave. Therefore, as is described in each application of the conventional technique, it is supposed that the conventional high-frequency amplifier is used for the purpose of selecting one mode from a high output mode and a low output mode in a transmitter, and the conventional high-frequency amplifier is not used for the purpose of changing the load impedance in accordance with a time change of the envelope of the high-frequency signal.
Also, even though the change-over of the switch is performed at high speed, it is difficult to obtain a switch and a variable element which have characteristics of electric power resistance sufficiently possible to be used for an output circuit of a high-frequency amplifier and are operated at low loss. As a result, a problem has arisen that an effect of the improvement of the efficiency is not sufficient.
The present invention is provided to solve the above-described problem, and the object of the present invention is to provide a high-frequency amplifier in which an output load impedance is changed in accordance with a time change of an envelope based on a modulation wave and which has a sufficiently high saturation electric power and excellent efficiency.
In other words, the object of the present invention is to provide a high-frequency amplifier in which an operation is performed at high efficiency even in an operation state of a high back-off, that is, even in an operation state of an output electric power lower than a saturation electric power.
Also, the object of the present invention is to provide a feed-forward amplifier and a distortion compensating amplifier in which an operation is performed at a high saturation electric power and an excellent efficiency.